1. Field of the Invention
The present invention relates to an optical pickup device for reproducing and recording information on an optical recording medium by projecting a light beam toward the optical recording medium. In particular, the present invention relates to reductions of size and cost of the optical pickup device having a liquid crystal element for compensating for a wave aberration.
2. Description of Related Art
Recent years, optical recording medium such as a compact disk (hereinafter referred to as a CD) or a digital versatile disk (hereinafter referred to as a DVD) has become commonplace and is used widely. Furthermore, in order to increase a quantity of information recorded on the optical recording medium, researches on the high density of the optical recording medium are being carried on. For example, a high density optical recording medium such as an HD-DVD or a Blu-Ray Disk (Registered Trademark; hereinafter referred to as a BD) is being available as a high quality DVD.
When information is reproduced or recorded on the optical recording medium, the optical pickup device is used for projecting a light beam toward the optical recording medium so that information can be recorded or be reproduced. The optical pickup device includes an objective lens having an appropriate numerical aperture (NA) and a light source having an appropriate wavelength in accordance with the type of the optical recording medium. For example, the objective lens for the CD has an NA of 0.50, and the light source for a CD has a wavelength of 780 nm. The objective lens for the DVD has an NA of 0.65, and the light source for the DVD has a wavelength of 650 nm. The objective lens from the HD-DVD has an NA of 0.65, and the light source for the HD-DVD has a wavelength of 405 nm. The objective lens for the BD has an NA of 0.85, and the light source has a wavelength of 405 nm.
Since the NA of the objective lens as well as the wavelength varies in accordance with the type of the optical recording medium as described above, different optical pickup devices may be used for different optical recording media. However, it is more convenient if a single optical pickup device can read or record information on a plurality of types of optical recording media. Such optical pickup devices have been developed in quantity. Among them, there is an optical pickup device disclosed in JP-A-2005-71424, for example, in which a single objective lens is used for reproducing and recording information on a plurality of types of optical recording media.
When supporting a plurality of types of optical recording media by using a single objective lens, even if the objective lens is adjusted so that a spherical aberration is not generated in one type of the optical recording medium for example, the spherical aberration may be generated when reproducing or recording information on other types of the optical recording media. Therefore, as shown in JP-A-2005-71424, it is common to arrange a liquid crystal panel in the optical pickup device and to control the voltage that is applied to the liquid crystal panel so that the spherical aberration is compensated. As the liquid crystal panel that is disposed for this purpose, a following type is described in JP-A-2005-71424 and other many documents, for example. In the type, a transparent electrode constituting the liquid crystal panel is divided into a plurality of areas in a concentric manner, and voltages applied to the areas are controlled so that the spherical aberration is compensated.
However, when the above-mentioned liquid crystal panel is used, it is necessary to increase the number of division of the concentric areas of the transparent electrode in order to compensate for the aberrations that are generated for the plurality of types of optical recording media. As a result, the number of electrodes and the number of wiring connections provided to the circuit board for controlling the liquid crystal panel are increased, and a size as well as a manufacturing cost of the optical pickup device is increased. Concerning this point, it is possible not to compensate for the aberration of the plurality of types of optical recording media completely but to compensate in general for the aberrations so that the number of electrodes provided to the circuit board can be reduced to a certain number, but there is a limitation in the reduction.
In addition, it is possible to reduce the number of electrodes and the number of wiring connections provided to the circuit board for controlling the liquid crystal panel by using a segment type liquid crystal panel that includes a plurality of transparent electrodes having a low resistance arranged in concentric manner and has neighboring electrodes connected to each other via a resister element having the same material as the transparent electrode, or by using a gradation type liquid crystal panel having a plurality of concentric electrodes having a low resistance on a transparent electrode of high resistance that is not divided, as shown in JP-A-2005-71424. In the case where this structure is adopted, however, the structure of the liquid crystal panel will become complicated in the first place. Furthermore, a manufacturing cost of the liquid crystal panel will be increased.